Some of electrode assemblies for an electric storage element such as a battery including a non-aqueous electrolyte secondary battery and a capacitor have structure in which a positive sheet, a negative sheet, and two separators, all of which has a strip-like shape, are stacked and wound. Active material layers are formed on both surfaces of the positive sheet and the negative sheet. On one ends in a width direction of the positive and negative sheets, active material layers are not formed so as to expose the metal foils to form leads. The separators are respectively interposed between the positive sheet and the negative sheet. The lead protrudes from any one of the both ends in the width direction of the separators.
An electrode assembly included in a secondary battery disclosed in JP No. 2011-216399 A includes a joint portion that joints the two separators at one end on an opposite side with respect to the lead the positive sheet and/or the negative sheet in the width direction. The joint portion is formed by, e.g., thermal fusion bonding of the two separators by heat rollers at the time of winding. By forming the joint portion, the positive sheet and/or the negative sheet are wrapped with the two separators at one end on the opposite side with respect to the lead, resulting in that foreign material is prevented from being in contact with the portions. The foreign material includes, e.g., metal particles generated when the leads of the positive sheet and/or the negative sheet are welded to an electric collector by ultrasonic welding.